Indium tin oxide (ITO) film has been known as a representative transparent conducting oxide for organic light-emitting diodes, liquid crystal display, and solar cell application. The extension of ITO from the passive electrode to an active channel layer for transparent field-effect transistors (FETs), however, has been largely limited due to its high carrier density (>10^20 cm-3), wide-band gap, and polycrystalline nature. Here, we demonstrate that the control of cation composition in ITO-based oxide films via solid doping of titanium (Ti), can optimize the carrier concentration as well as the suppression of film crystallization. On 120 nm thick SiO2/Mo (200 nm)/glass substrate, transparent n-type FETs containing approximately 4 at.% Ti-doped ITO film, fabricated by co-sputtering of ITO and TiO2, exhibit high electron mobility up to 13.4 cm2V-1s-1, low subthreshold gate swing of 0.25 Vdecade-1, and high Ion/Ioff ratio of >10^8.